1. FIELD OF THE INVENTION
This invention relates to a refractive lens system and, more specifically, to a wide spectral band refractive lens capable of operation in the 3 to 13 micron infrared ranges, providing for simultaneous use of both the 3 to 5 and 8 to 12 micron atmospheric windows.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Prior art refractive lens systems which operate in the infrared frequency range are generally restricted to operation in either the 3 to 5 micron range or the 8 to 12 micron range, but not both. This is due to the fact that prior art lens designs are not adequate to operate in both ranges. The reason for this problem is that the refractive indices of the lens materials vary from one band to the other for different frequencies, thereby providing a color problem as the operating frequency changes. In view of this problem, the prior art systems which have been capable of operation in both infrared bands have utilized reflective or mirror-type system whereby the color problem is removed. Reflective optics have some disadvantages. For example, large fields of view cannot be obtained. Also, central obscurations limit the diffraction modulation transfer function (MTF) in the 8 to 12 micron band. Furthermore, there are difficulties in integrating reflective optics with large fields of regard, azimuth and elevation, search mirrors and derotation prisms. It is therefore apparent that a system has been long sought which provides the advantages of refractive optics, yet is capable of operation in both the 3 to 5 and 8 to 12 micron ranges simultaneously, as is the case with reflective optics.